The present invention relates to an opto-electronic system for point by point measurement of spatial coordinates. More specifically, the invention relates to a method and system for point by point measurement of spatial coordinates, where a touch probe comprising a minimum of three point-sized light sources at known coordinates relative to a local probe fixed coordinate system and with a touch point at a known location relative to the local coordinate system, is brought into contact with the point for which the coordinates are to be determined.
The system is based on the use of opto-electronic angle sensors calibrated for measurement of angles in two dimensions (spatial direction) towards active light sources or diffuse light reflexes, e.g. as described by the inventors in Norwegian Patent No. 165046.
The present invention proposes a system solution using one angle sensor in combination with a measurement probe as described by the inventors in Norwegian Patent Application No. 901891, and in Swedish Patent no. 456 454.
A system according to the invention described in Norwegian Patent no. 165046 allows registration with high precision of the position, orientation and/or surface geometry of objects, either static or dynamic. This is only to a limited extent possible by existing non-contact measurement techniques. The flexibility and transportability of the system makes it applicable for measurement tasks that can not be solved by conventional mechanical coordinate measurement machines. Such machines are large and complex, expensive and less flexible. The system is optimized for high accuracy.
As one angle sensor alone gives the information of the direction towards a point only, conventionally two or more angle sensors are used in combination. The spatial coordinates of a point are calculated by a so called intersection technique. Starting from known coordinates of the angle sensors, as well as the measured spatial directions, the coordinates are calculated for the point where the lines of sight from the individual angle sensors intersect. In the case of using two angle sensors the intersection angle shall be as close to 90 degrees as possible to obtain optimum accuracy in all three dimensions (x, y, z). This introduces high requirements for free sight, since all measurement points have to be seen from at least two different directions. This may be a problem in industrial applications, since the object to be measured is often partly or completely hidden by robots or other production equipment.
The present further development suggests a simplified system based on one angle sensor and a specially made touch probe. Spatial coordinates can be determined by the use of only one angle sensor, if the touch probe is equipped with a minimum of three measurement points at known locations relative to the touch point of the probe. The use of a touch probe eliminates sighting problems, both by the fact that the measurement point itself must not be seen by the angle sensor as long as all of the measurement points of the probe are seen, and that the line of sight requirements are reduced to that of one angle sensor only. This leads to a simplified setup of the angle sensor, better access to difficult areas, better transportability, and increased measurement speed.
Normally, a system based on one angle sensor only, will not give the same accuracy in spatial coordinates as systems based on a plurality of angle sensors of corresponding type. Especially this is the case for the direction of depth relative to the angle sensor. However, there are a number of geometry measurement tasks where the flexibility and measurement speed requirements are higher than the accuracy requirements. Furthermore, there are a number of industrial measurement problems where a high accuracy is required in two dimensions, but less in depth, e.g. the determination of the straightness and roundness of an aircraft fuselage.
Norwegian Patent No. 165046 describes a fully automatic and accurately calibrated angle sensor as shown in FIG. 1. That sensor is developed to measure the direction towards points like active light sources or points illuminated by active light sources. This ensures secure measurement point identification, and hence allows automatic operation, as well as ensures a very high signal to noise ratio and hence contributes to high accuracy.
The angle sensor mainly comprises a camera housing 1, a lens unit 2 and a two dimensional array (matrix) 3 of photosensitive elements 5. The lens unit is a camera lens with standard, spherical optics, having a focal distance mainly given by the field of view requirements. Possibly, the lens may have an anti-reflection coating or optical filter, which has to be matched to the spectral distribution of the light sources to be used. The photosensitive elements may e.g., be of CCD (Charge Coupled Device) or CID (Charge Injected Device) type. Due to the accuracy requirements, normally matrices of maximum resolution will be applied. If the system speed is of main importance, matrices having fewer elements will be applied. High accuracy is ensured by the use of accurate procedures to calibrate the angle sensor. This is described in Norwegian Patent No. 165046.
FIG. 2 shows the principle for spatial direction measurements. The fully automatic function of the angles sensor is based on the use of active light sources, e.g., light-emitting diodes 6. The image of the light-emitting point 5 given by the lens system 2 is an illuminated spot 7 on the array of photosensitive elements 3. The image illuminates a number of elements 5 with an intensity distribution given by the size of the emitting point, and the resolution of the lens system. The position of the illuminated spot is an unambiguous measure of the spatial direction towards the imaged point. The spatial direction is given as two angles .alpha. and .beta.. .beta. is the angle between the spatial direction and the horizontal plane of symmetry of the angle sensor, .alpha. is the angle between the optical-axis and the direction towards the projection of the light-emitting point into the horizontal plane of symmetry. Both angles .alpha. and .beta. have values 0 at the optical axis.